Indications and Clinical Considerations
Omohyoid syndrome is caused by trauma to the fibers of the inferior belly of the omohyoid muscle. The syndrome is seen most often in patients who have recently experienced a bout of intense vomiting or sustained a flexion–extension injury to the cervical spine and to the musculature of the anterior neck. Concurrent trauma to the brachial plexus with upper extremity symptomatology also may accompany trauma-induced omohyoid syndrome. The pain of omohyoid syndrome manifests as myofascial pain. It is constant and exacerbated with movement of the affected muscle. A trigger point in the inferior belly of the omohyoid muscle is often present and provides a basis for treatment. The pain starts just above the clavicle at the lateral aspect of the clavicular attachment of the sternocleidomastoid muscle. The pain may radiate into the anterolateral neck. Injection of the trigger point in the inferior muscle of the omohyoid muscle with local anesthetic and corticosteroid serves as both a diagnostic and a therapeutic maneuver.
Clinically Relevant Anatomy
The omohyoid muscle extends from the hyoid bone laterally and inferiorly to the insertion at the upper margin of the scapula ( Figure 15-1 ). The intermediate tendon of the omohyoid muscle that runs from the muscle inferiorly to attach at the clavicle tethers the muscle down, and the point of musculotendinous insertion is susceptible to trauma. The inferior portion of the omohyoid muscle is further confined by the overlying attachment of the sternocleidomastoid to the clavicle. The omohyoid muscle also is susceptible to trauma at this point. The internal jugular vein and common carotid artery lie deep to the omohyoid muscle, and the brachial plexus lies more lateral.
The key landmark for injection when treating omohyoid syndrome is the lateral aspect of the clavicular head of the sternocleidomastoid muscle (see Figure 15-1 ). The omohyoid muscle is located slightly lateral and deep to the clavicular head of the sternocleidomastoid muscle ½ to 1 inch above the superior margin of the clavicle. Given the relationship of the great vessels of the neck to the omohyoid muscle, care must be taken when placing needles in this anatomic area.
The patient is placed in a supine position with the head turned away from the side to be blocked. A total of 3 mL of local anesthetic is drawn up in a 5-mL sterile syringe. When omohyoid syndrome is treated, a total of 80 mg of depot corticosteroid is added to the local anesthetic with the first block, and 40 mg of depot corticosteroid is added with subsequent blocks.
The patient is then asked to raise his or her head against the resistance of the pain specialist’s hand to aid in identification of the posterior border of the sternocleidomastoid muscle. The point at which the lateral border of the sternocleidomastoid attaches to the clavicle is then identified. At this point, slightly lateral and approximately 1 inch above the clavicle, after preparation of the skin with antiseptic solution, a 1½-inch needle is inserted directly perpendicular to the tabletop (see Figure 15-1 ). The needle should be advanced quite slowly because of the proximity of the great vessels and brachial plexus. A pop is often felt as the fascia of the omohyoid muscle is pierced. This should occur at a depth of ½ to ¾ inch. If strict attention to technique is observed and the needle has not been placed or directed too laterally, the brachial plexus should not be encountered. However, because of the proximity of the brachial plexus, the patient should be warned that a paresthesia could occur and to say “there!” should a paresthesia be felt. The needle should never be directed in a more inferomedial trajectory, or pneumothorax is likely to occur.
After the muscle has been identified, gentle aspiration is performed to identify blood or cerebrospinal fluid. If the aspiration test result is negative and no paresthesia into the distribution of the brachial plexus is encountered, 3 mL of solution is slowly injected, with the patient monitored closely for signs of local anesthetic toxicity or inadvertent neuraxial injection.